Law Dome is a special spot in eastern Antarctica where scientists have been drilling down into the continent’s long-frozen surface to pull out cores of ice.

Trapped in the ice cores are bubbles that give a record of the composition of the Earth’s atmosphere going back tens of thousands of years.

New analysis of those bubbles by a group of Australian, British and Italian scientists could have just given an answer to a crucial question.

As the Earth gets warmer, how will the planet’s soils and plants react? Will they start to “burp” too much CO2 and, if so, how much?

Answering this question is crucial as scientists try and work out just how much more greenhouse gases humans can afford to emit while still keeping to temperature targets agreed at the Paris climate talks last year.

The new ice core analysis, published in the journal Nature Geoscience, suggests the news is less than encouraging.

The research finds that for every 1C of warming, the Earth’s plants and soils – the “terrestrial biosphere” – will add carbon dioxide to the atmosphere to the tune of about 20 parts per million. Levels in the atmosphere continue to climb.

But to arrive at this number, the scientific team went back to analyse bubbles trapped in the ice during the Little Ice Age (LIA) – a period which started around the year 1500 and lasted for about 250 years.

The ice cores used in the study were drilled from the late 1980s to the mid 2000s and are stored near CSIRO’s Aspendale lab in Victoria and at the Australian Antarctic Division in Tasmania. The study also used data from ice cores drilled by the British Antarctic Survey 20 years ago.

Etheridge, a principal research scientist at CSIRO and co-author of the new research, has been to Antarctica seven times and is a veteran of ice core drilling programs.

Working temperatures for scientists at Law Dome can range from a balmy minus 5C (Etheridge claims T-shirts are worn. I don’t believe him) to a blustery -20C to a clear but frigid 50 degrees below freezing (that’s -58F).

Little Ice Age

During the LIA, temperatures globally cooled by a half degree Celsius or so and the amount of carbon dioxide in the atmosphere dropped.

Etheridge says: “Before the industrial revolution there were changes in the global temperature and we have observations of that from the ice cores. This Little Ice Age was a natural event triggered probably by volcanoes going off and a slightly weaker sun.”

Scientists have known that atmospheric CO2 dropped during the LIA, but were not sure why.

Bubbles trapped in a slice of an Antarctic ice core Photograph: David Etheridge

One theory was that as pandemics swept through the Americas and Europe, this reduced the amount of farming activity. In turn, the regrowth sucked more CO2 back into soils and plants.

But by analysing another compound trapped in ice cores – carbonyl sulfide – the scientists were able to show it was the falling temperatures, rather than the regrowth of plants, that caused the terrestrial biosphere to suck up more CO2 (and in turn, cut atmospheric levels of carbon dioxide).

If this were to operate at this level in the future it would significantly reduce the available carbon budget

Dr Bill Hare

On the flip side, this now means that as the planet warms, the Earth’s plants and soils will likely store less carbon and help push up CO2 levels in the atmosphere.

Essentially, Etheridge and his colleagues have found that the Earth’s terrestrial biosphere acts as a “positive feedback” in the climate system as global warming continues.

“More warming, leads to more CO2 from the terrestrial biosphere, which leads to more warming,” says Etheridge.

Etheridge explained there was some good and bad news from the study.

The good news was that the findings suggested that some of the worse predictions of a potential feedback from plants and soils were probably now ruled out. “We now have a number to work with,” he said.

But the bad news was that the “positive feedback” was now confirmed, ruling out other studies that had predicted lower amounts of CO2 being released from the terrestrial biosphere, or not released at all.

Carbon cycle

This flow of carbon between the Earth’s atmosphere and its plants and soils is a vital issue for scientists to understand.

Scientists have started to set “budgets” for the amount of carbon dioxide that can be released by humans while still have a shot at keeping warming well below 2C.

If the terrestrial biosphere adds more CO2 to the system than models anticipate, then this could mean governments around the world have less in that carbon budget than they thought.

Climate scientist Dr Bill Hare, founder of the respected group Climate Analytics, has been a key adviser to governments and the United Nations on carbon budgets.

I asked him for his view on the Nature Geoscience paper, which he described as “elegant” (I’ve found this is a word scientists often use when they’re mightily impressed with someone else’s work).

Hare said: “The paper shows a significant positive feedback from warming on the terrestrial carbon cycle. If this were to operate at this level in the future it would significantly reduce the available carbon budget for fossil fuel CO2 emissions, compared to present mid range estimates.”

Hare said the findings would “tend to push the budgets towards the lower range of the estimates available at present”.

“If there is a positive feedback as the authors calculate, this would only add to the urgency of reducing emissions as rapidly as possible in order to minimise that feedback and to minimise the impact on the remaining carbon budget.”

Roughly translated, all of this means that ice bubbles from the frigid Antarctic have just made the job of cutting fossil fuel emissions even more urgent.